Postfix: how to force MX lookup for 'local' domain?

Let's say my server's hostname is site.com. I also run various web based software packages, which send out e-mail to customers. This works great. The problem is that my server also sends out e-mail to user@site.com.

The problem is that the site.com e-mail is not hosted on this server, it's actually hosted on a 3rd party e-mail server, so I am not getting these e-mails. Is there a way of forcing Postfix to use the MX records for site.com, even if this is the local name of the server? The server will not be receiving ANY e-mail, just send out.

Oops, I didn't realize I was in the ISPConfig2 subforum, this is about a clean postfix install on Ubuntu 10.04 LTS. Hopefully a mod can move this thread.

I am hoping I can avoid hardcoding the SMTP server, or I will run into problems when the 3rd party changes its MX records. I just want to force postfix to look up the MX records for site.com whenever e-mail is sent to site.com.